Aerosol generation systems are used in industry for injecting powders into tubes for pneumatic transport; in toxicology and industrial hygiene for generating study atmospheres; and in medicine for delivering particulate drugs to patients by the inhalation route. Several types of generators are commercially available, including: the Venturi tube powder injector, which utilizes pressurized air (Bohnet M., Calculation and design of gas/solid injectors. Powder Technology, 302–313, 1984; Cheng, Y., Barr et al., “A Venturi dispenser as a dry powder generator for inhalation studies,” Inhalation Toxicology 1: 365–371, 1989), the Wright dust feeder, which uses a rotating scraper, (Wright, B., “A new dust-feed mechanism,” Journal of Scientific Instruments, 27: 12–15, 1950) and various fluidized bed designs, which use blowing air (Drew, R. and Laskin, S., “A new dust-generating system for inhalation studies,” American Industrial Hygiene Association Journal, 32: 327–330, 1971; Ebens R. and Vos, M. “A device for the continuous metering of small dust quantities,” Staub-Reinhalt der Luft, 28(5): 24–25, 1971).
Typically, a jet pump or ejector is used to aspirate powder from a powder container or hopper and to transfer the powder through an outlet conduit to a spray device (U.S. Pat. No. 5,056,720). The powder can be supplied from a fluidized bed (U.S. Pat. No. 3,746,254). In order to meter or control the rate of flow of powder from the powder source, such pumps conventionally include a metering air flow that injects a controlled flow of air into the powder supply. The pressure of this metering air flow controls the amount of air which is mixed with the powder entering the pump. Consequently, if the metering air flow pressure is increased, the amount of air in the powder-air mixture is increased and therefore the net powder flow rate is decreased. Conversely, if the metering air flow pressure is decreased, the amount of air in the powder-air mixture is decreased and therefore the net flow rate of powder is increased.
One of the shortcomings of these powder delivery systems is that the powder ejected normally flows unevenly to the target. There are periodic puffs or clouds of powder delivered and periodic reductions in the density of powder delivered. Such periodic increases or decreases in powder density result in uneven application of powder to the target, which are not desirable. Thus the ability to control the flow rate of the powder is very important in order to deliver the powder smoothly without surging or pulsing effects; to control the velocity at which the powder is delivered; and to insure that the air entrained powder is well dispersed in the air stream before reaching the target, such as a reaction chamber for the production of carbon nanotubes. Thus, there is a need for methods for delivering powders in a controlled manner.